The present invention relates generally to blast furnaces for smelting iron ore, and more particularly to a blast furnace tuyere having a replaceable liner.
A blast furnace is a shaft-type furnace into the top of which are introduced iron ore, coke and limestone and into a lower hearth portion of which is introduced a blast of hot air, to perform the smelting operation. The blast of hot air is preheated in auxiliary stoves to a temperature in the range 1600.degree.-2200.degree. F. (871.degree.-1204.degree. C.) and introduced into the furnace proper through a plurality of tubular elements or nozzles called tuyeres. The tuyere is usually composed of copper and is cooled by water circulated through the tuyere to maintain the tuyere at a temperature of about 400.degree.-600.degree. F. (204.degree.-315.degree. C.).
A substantial amount of heat is lost from the hot air blast as it passes through the water-cooled tuyeres. There has been a substantial increase in blast furnace productivity over the years, through various improvements in blast furnace design and operation, and the increased productivity has required an increase in the quantity and circulation rate of cooling water circulated through the tuyeres. This in turn produces increased heat loss from the hot air blast as it passes through the tuyeres. This heat loss is undesirable because it reduces the operating temperature within the blast furnace.
A blast furnace may operate at flame temperatures in the range 3500.degree.-4000.degree. F. (1927.degree.-2204.degree. C.), for example. This is the temperature inside the blast furnace, in front of or inwardly of the tuyeres. There is an optimum operating flame temperature for a blast furnace, depending upon the make-up of the raw materials therein. If the actual temperature within the blast furnace drops below the optimum operating temperature, coke consumption must be increased to raise the temperature back to optimum, resulting in a substantial increase in operating expense. An alternative is to increase the temperature of the hot air blast upstream of the blast furnace tuyeres to compensate for the loss in heat resulting from the passage of the hot air blast through the water-cooled tuyeres. This, however, increases fuel consumption at the stoves in which the air blast is heated, and it also excessively increases the amount of maintenance required for refractory linings, valves, expansion joints, etc. in the equipment in which the hot air blast is transported from the stoves to the tuyeres.
Attempts have been made in the past to line the inside surface of the tuyere with a refractory material, and this has decreased the heat loss in the hot air blast. However, these refractory linings had drawbacks.
More particularly, in one arrangement, the inside surface of the tuyere was lined with a porous refractory material such as a castable material or a ramming mix. This resulted in a 25-30% decrease in the heat loss experienced with an unlined tuyere. However, the porous refractory material would not stay in place and had to be replaced very frequently, and this required removal of the tuyere from the furnace in turn requiring back drafting of the furnace. The resultant loss of furnace processing time offset any savings in coke consumption achieved by the utilization of the porous refractory lining on the inside surface of the tuyere.
Another arrangement employed a hard refractory liner in the form of a ceramic tube (e.g., composed of silicon carbide) which fit inside the tuyere, extended essentially the full length of the tuyere, and utilized a single layer of refractory fiber paper between the ceramic tube and the inside surface of the tuyere as a seating for the ceramic tube. This produced a 35-40% reduction in heat loss, but this arrangement also had a relatively short life in that the nose or inner end of the ceramic liner was exposed to the blast furnace interior and the liner wore away from the nose outwardly in a relatively short time. This produced the same replacement problem as did the use of porous refractory material.
There was an arrangement which attempted to cope with the problem of refractory wear initiating at the nose of the liner. That arrangement employed a recess in the inside surface of the tuyere. The recess had an inner end spaced in an outward direction from the nose of the tuyere, and a shortened silicon carbide liner was seated in the recess. The nose of the shortened liner was not directly exposed to the blast furnace interior but was protected therefrom by the tuyere wall at the inner end of the recess. This, however, decreased the reduction in heat loss down to about 25% compared to a 35-40% reduction when the silicon carbide liner extended the full length of the tuyere.
Still another arrangement employed a full length silicon carbide tube utilizing a seating composed of castable refractory material arranged in a layer between the silicon carbide tube and the inner surface of the tuyere. This produced a reduction in heat loss of only about 25-30%.
Refractory fiber paper has excellent insulating properties, but it is incapable of being self-retained along the inside surface of the tuyere.